Accessing stored environment variables is now done through the @metadata field:

["@metadata"]["field_name"]

This would reference field field_name, which in the above example references
the ENV_VAR_NAME environment variable.

Previous versions of this plugin put the environment variables as
fields at the root level of the event. Current versions make use of the
@metadata field, as outlined. You have to change add_field_from_env in
the older versions to add_metadata_from_env in the newer version.

If the event has field "somefield" == "hello" this filter, on success,
would add field foo_hello if it is present, with the
value above and the %{host} piece replaced with that value from the
event. The second example would also add a hardcoded field.